This invention relates generally to pulse modulator circuits and is particularly adapted for grid control of traveling wave tubes (TWT).
Many modern pulse type radar transmitters use grid controlled TWT radio frequency amplifiers in a configuration such that the grid of the TWT is at a potential of from 35,000 to 100,000 volts with respect to ground. Further, it is desired that the width of the pulse applied to the grid of the TWT be substantially the same as that of a low level pulse supplied from a radar signal processor. Of the several types of TWT grid pulse modulators presently in use many implement "ON and OFF" switching arrangements at the high voltage level of the TWT. As could be anticipated such arrangements are relatively complex to mechanize and have inherent reliability problems.
Further, such direct coupled pulse modulators have problems producing a pulse with a fast trailing edge. To alleviate this problem prior modulators have incorporated "tail biter" circuits. Said last mentioned circuits normally operate by shorting the output of the pulse modulator to ground at the time when it is desired to terminate the pulse; and although such an approach is effective in decreasing the fall time of the trailing edge of the pulse, if timing misalignment occurs in the circuit considerable energy is shorted to ground and the life of the pulse modulator can be substantially reduced.
Also, it has been Applicant's experience that prior art direct coupled modulators which contain complex semiconductor circuitry operating at the high voltage potentials of TWT circuits suffer from reliability problems since they are vulnerable to TWT arcing and high voltage power supply "crowbarring".